Even though this might seem to be an old Report of Human Rights Situation in Nigeria compiled by the United Nations in 1996. It is still interesting to note that the report actually claimed that the Yoruba are in the South East, while the Ibo in the South West as you can see in the quote below.

C. Historical background

6. According to a 1995 report of the United Nations Development Programme,
Nigeria has a population of 102 million, composed of some 400 ethnic,
religious or linguistic groupings. The four main groupings are the Hausa and
the Fulani in the north, the Yoruba in the south-east and the Ibo in the
south-west. In what is now known as the Federal Republic of Nigeria, there
have been many ethnic conflicts and inter-ethnic tensions.

http://www.un.org/documents/ga/docs/51/plenary/a51-538.htm